After decades of litigation over policies of racial discrimination, the practice still exists in some major American labor unions. Local 28 of the Sheet Metal Workers in New York City advertises “Opportunity, Diversity and Middle Class Careers.” But pictures of blacks, whites and Hispanics working closely together obscure a “stark racial divide,” according to Rachel Swarns of the New York Times in a story about a legal case first filed by the Justice Department 44 years ago.